What Controls the Interannual Variability of the Boreal Winter Atmospheric River Activities over the Northern Hemisphere?

Interannual variability of the winter AR activities over the Northern hemisphere is investigated. The leading modes of AR variability over the North Pacific and North Atlantic are first identified and characterized. Over the Pacific, the first mode is characterized by a dipole structure with enhanced AR frequency along the AR peak region at about 30° N and reduced AR frequency further north. The second mode exhibits a tri-pole structure with a narrow band of positive AR anomalies at about 30° N and sandwiched by negative anomalies. Over the Atlantic, the first mode exhibits an equatorward shift of the ARs with positive anomalies and negative anomalies located on the equatorward and poleward side of the AR peak region at about 40° N , respectively. The second mode is associated with the strengthening and eastward extension of the AR peak region which is sandwiched by negative anomalies. A large ensemble of atmospheric global climate models from the Coupled Model Intercomparison Project phase 6 (CMIP6), which shows high skills in simulating these modes, is then used to quantify the roles of sea surface temperature (SST) forcing versus internal atmospheric variability in driving the formation of these modes. Results show that SST forcing explains about half of the variance for the Pacific leading modes, while that number drops to about a quarter for the Atlantic leading modes, suggesting higher predictability for the Pacific AR variability. Additional ensemble driven only by observed tropical SST is further utilized to demonstrate the more important role that tropical SST plays in controlling the Pacific AR variability while both tropical and extratropical SST exert comparable influences on the Atlantic AR variability.

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